Tough VSS Question about Compatability

[section525]

Hi Everyone! I cam over from the Explorer Forum with a very tricky and technical VSS question which has yet to be answered by the four or five forums I've posted it in.

I'm hoping there's a VSS genius here at this forum.

My situation:

I am swapping the body of my '96 V8 2WD Explorer onto the chassis of a '99 V8 AWD Explorer. Sounds easy enough. The problem I am having is the VSS are different (maybe) and are in different places.

On my '96 2WD, the VSS is on the tranny tailshaft housing (if it were AWD it would be on the transfer case). In 1998 Ford changed the VSS location to the rear axle.

I am assuming that the VSS's are different between the two years. It would be nice if I could just connect my tail shaft wiring to the rear axle one of the '99. But I think that sounds too easy.

Does anyone know if the EEC-V on my '96 can read and interpret the signal from the '99 axle VSS? I know the VSS uses a Hall Effect to send a digital ouput. I'm thinking that if the two outputs are different, I can get a Speed Cal or something of the kind to adjust the output readings so my '96 EEC-V knows the correct speed of the vehicle.

Thanks for the help and reading my looong post!

[ivymike1031]

on the later model vehicle, is the sensor reading the differential input, or one of the axle shafts?

are the diff ratios the same?

[section525]

I'm assuming it is in the differintial, since I saw no other harnesses going to the axle anywhere. Both axles have 3.73s too!

[ivymike1031]

I've seen some service manuals that give detailed specs on the voltage output of the VSS, but I don't know about the manual for your vehicle. If the diff ratios are the same, and the VSS reads a shaft that spins at the same speed as the trans output (96 is same speed as 99), then the only remaining question is how the electronic signal compares. In the absence of an o-scope or similar equipment, I'd just hook it up and see if it works... If the '96 computer can't figure out what the '99 sensor is saying, then you can possibly adapt the '96 sensor to mount in the location of the '99 sensor, or something along those lines. You could also go the route you mentioned, and modify the electronic signal. It all depends on how the geom & signal compare, though.

oh drat, I just thought of a possible problem with my earlier statement about the shaft speed... it's not the shaft speed that really matters, but the frequency of the "bumps" on the shaft. If the shafts turn at the same speed, but have different numbers of bumps, the signals would have different frequencies (different speed readings, in other words).

[section525]

This is what someone said about it:

I've done a little research of my '95 and '99 manuals, and the best I can figure out is my first guess was incorrect. It seems that that the VSS and the RABS may be quite similar. They both generate an AC signal which increases in frequency as the vehicle speed increases. It appears that the frequency is the same between them (65 Hz @ 30 MPH and 125 Hz @ 55 MPH). However, in the testing procedeure, it states that the VSS should generate a voltage between 1.0-5.0 V, while the RABS voltage should be between 100 mV-3.5V. Even though the RABS is producing the correct frequency, the lower voltage may not be strong enough to produce a trouble-free signal for your situation.

Edit: Something doesn't seem right, but I can't put my finger on it. The '99 lists the frequency given above as a "VSS" signal, but it doesn't have a VSS. The signal from the RABS is input to the 4WABS, then the 4WABS sends a speed signal to all the various components which require it. I don't know if the frequency given is the raw RABS signal, or the output from the 4WABS. My gut feeling is the frequency from the RABS is much higher (if I remember correctly, the toothed ring the RABS uses to generate a signal has 120 teeth) then the programming that is done to the computer correlates this raw signal to the frequencies required for the VSS signal. I do know that the more I look at it, the more confused I become.

Maybe you can help interpret it for me! Thanks!